One who breaks security on a system. Coined ca. 1985 by hackers in defense against journalistic misuse of hacker (q.v., sense 8). An earlier attempt to establish `worm' in this sense around 1981-82 on Usenet was largely a failure.
Use of both these neologisms reflects a strong revulsion against the theft and vandalism perpetrated by cracking rings. The neologism "cracker" in this sense may have been influenced not so much by the term "safe-cracker" as by the non-jargon term "cracker", which in Middle English meant an obnoxious person (e.g., "What cracker is this same that deafs our ears / With this abundance of superfluous breath?" - Shakespeare's King John, Act II, Scene I) and in modern colloquial American English survives as a barely gentler synonym for "white trash".
While it is expected that any real hacker will have done some playful cracking and knows many of the basic techniques, anyone past larval stage is expected to have outgrown the desire to do so except for immediate, benign, practical reasons (for example, if it's necessary to get around some security in order to get some work done).
Thus, there is far less overlap between hackerdom and crackerdom than the mundane reader misled by sensationalistic journalism might expect. Crackers tend to gather in small, tight-knit, very secretive groups that have little overlap with the huge, open poly-culture this lexicon describes; though crackers often like to describe themselves as hackers, most true hackers consider them a separate and lower form of life.
Ethical considerations aside, hackers figure that anyone who can't imagine a more interesting way to play with their computers than breaking into someone else's has to be pretty losing. Some other reasons crackers are looked down on are discussed in the entries on cracking and phreaking. See also samurai, dark-side hacker, and hacker ethic. For a portrait of the typical teenage cracker, see warez d00dz.